Is it harmful if a man performs oral on a woman with a yeast infection? could he get thrush

Possibly: Candida is an opportunistic pathogen in the mouth, usually rearing it's ugly head after antibiotics have altered the natural flora which usually keeps it in check. In theory, if a man had some decrease in his regular immune status, or was on antibiotic therapy, it could happen. Try using a douche made of plain yogurt and warm water several hours before sex if possible- it may help kill off yeast.

Yes: This is entirely possible. I would suggest waiting until the infection has resolved.

No: Studies do, however suggest that application of saliva to the vulvovaginal area may "disrupt the balance" of candida to other resident organisms and immune components, predisposing you to candidal/yeast infections. Any risk for you or your partner may also be related to your immune system.